Our objective is to breed transgenic animals that are to be used by the investigators in the projects of this Program Project Grant entitled: "Murine Models of Human Immunodeficiencies". Several transgenic mice lines are already available for studies in core B. A number of these animals were generated at the Alt and Terhorst labs, others were obtained from colleagues for breeding purposes. Key mice that are already available include transgenic lines that lack T cells and NK cells (T-/NK- ), Rag-2 deficient mice which lack B immunodeficient mouse strains for the reasons stated in Project 1: Transgenic Models of T Cell Receptor Immunodeficiencies. COX TERHORST, Beth Israel Hospital. Project 2: HIGMX-1: Transgenic Models of CD40 and CD40L deficiency. Raif Geha and Fred rosen, Children's Hospital. Project 3: Murine XLA and SCID Models. Frederick ALT and Fred Rosen, The Center for blood Research. Specifically we propose to: 1. Set up rag-2 colony for far-2-/- blastocyst complementation studies in Projects 1, 2, and 3. 2. Breed transgenic mice generated in Projects 1, 2, and 3. 3. Generate novel mouse strains through interbreeding animals generated by the investigators in Projects 1, 2, and 3. 4. Conduct preliminary analyses of immunodeficient mice.